Curved spring structure with elongated section located under cantilevered section

ABSTRACT

A curved spring structure includes a base section extending parallel to the substrate surface, a curved cantilever section bent away from the substrate surface, and an elongated section extending from the base section along the substrate surface under the cantilevered section. The spring structure includes a spring finger formed from a self-bending material film (e.g., stress-engineered metal, bimorph/bimetallic) that is patterned and released. A cladding layer is then electroplated and/or electroless plated onto the spring finger for strength. The elongated section is formed from plating material deposited simultaneously with cladding layers. To promote the formation of the elongated section, a cementation layer is provided under the spring finger to facilitate electroplating, or the substrate surface is pre-treated to facilitate electroless plating.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/971,467, entitled “Curved Spring Structure With Elongated SectionLocated Under Cantilevered Section” filed Oct. 21, 2004.

FIELD OF THE INVENTION

This invention generally relates to curved micro-spring structuresformed from self-bending materials that are used, for example, as testprobes and interconnect structures for integrated circuits, and moreparticularly to curved micro-spring structures that are metal plated.

BACKGROUND OF THE INVENTION

Photolithographically patterned self-bending spring structures (e.g.,spring probes) have been developed, for example, to produce low costprobe cards and to provide electrical connections between integratedcircuits. A typical self-bending spring structure is formed from astress-engineered (a.k.a. “stressy”) metal film that is intentionallyfabricated such that its lower/upper portions have a higher internaltensile stress than its upper/lower portions. For example, a springbending away from a substrate surface has lower tensile stress in thelower portion than in the upper portion, thus producing an upward bend(note that all of the examples provided herein describe this stressgradient). In contrast, a downward bending spring may be produced byproviding a higher tensile stress in the lower portion than in the upperportion. The internal stress gradient is produced in thestress-engineered metal film by layering different metals having thedesired stress characteristics, or using a single metal by altering thefabrication parameters during deposition. The stress-engineered metalfilm is patterned to form islands that are secured to an underlyingsubstrate either directly or using an intermediate release materiallayer. When the release material (and/or underlying substrate) isselectively etched from beneath a first (free) portion, the free portionbends away from the substrate to relieve the internal stress, therebyproducing a spring structure that remains secured to the substrate by ananchor portion, but has a bent “free” (cantilevered) portion thatextends away from the substrate surface. The tip of the cantileveredportion may then be contacted with selected pads on an integratedcircuit, or curvature of the spring structure may be controlled to forma loop or other desired shape. In this manner, such spring structure maybe used in probe cards, for electrically bonding integrated circuits,circuit boards, and electrode arrays, and for producing other devicessuch as inductors, variable capacitors, and actuated mirrors. Examplesof such spring structures are disclosed in U.S. Pat. No. 3,842,189(Southgate) and U.S. Pat. No. 5,613,861 (Smith).

When used to form probe cards, such spring metal structures must exhibitsufficient stiffness to facilitate proper electrical connection betweenthe probe (spring metal finger) and a corresponding contact pad on thedevice-under-test. Most stressy metal spring probes produced byconventional methods are fabricated from sputtered or plated metal thatis approximately one micron thick, which produces only a nominalstiffness capable of resisting a force of 0.1 to 0.2 grams (gmf). Thesestressy metal spring probes may provide sufficient stiffness to probegold contact pads, but are not stiff enough to reliably probe aluminumpads. Gold pads can be readily probed with relatively weak spring probesbecause gold does not form a passivation layer that takes significantforce to puncture. However, aluminum pads form a passivation layer thatmust be punctured by the tip of the spring probe in order to facilitateproper electrical connection. To repeatedly achieve electrical contactto aluminum, which is required for many integrated circuit probe cardapplications, deflection of the probes within their elastic regionshould absorb an expected force of at least a few grams.

One method of increasing the stiffness of stressy metal springstructures is to increase its thickness by producing thicker stressymetal films. However, the release height of a spring structure isproportional to its stress gradient divided by the stressy metal filmthickness. This means that, by making the stressy metal film thicker,the release height is reduced. Of course, one can compensate for thisreduced release height by increasing the stress gradient, but there arepractical limits to how much stress can be induced, and the inducedstress often cannot be increased enough to compensate for a very thickstressy metal film. Therefore, the (thin) stressy metal film thicknessitself is mostly used to tune for a desired release height.

A more desirable approach to generating spring structures having ahigher stiffness is to form and release a relatively thin stressy metalstructure, and then thickening the structure using a plating process.Most uses for spring structures today utilize plating (a.k.a.,“cladding”) of the released springs for improving various springcharacteristics such as electrical conductivity, hardness and wearresistance. Plating a thick metal layer (a few microns) on the stressmetal film significantly increases probe stiffness, but could alsodecrease the maximum deflection. Maximum deflection is determined by theinitial lift height and the fracture limit of the structure. Laboratoryexperiments have shown thick electroplated stiffened springs break oryield when deflected a significant fraction of their initial liftheight. Failure typically occurs at the base (anchor portion) of thecantilevers, where plating formed either on the bottom surface of therelease spring or spontaneously plated onto the underlying substratesurface forms a wedge that acts as a stress-concentrating fulcrum to prythe base away from the underlying substrate as the structure isdeflected, resulting in “delamination” of the spring structure. This iscurrently a serious issue for the reliability of stressed-metalinterconnects. Thermocycling results have shown that the current springstructure is very sensitive to delamination. This wedge limits themaximum force of the resulting spring structure because it limits boththe allowed thickness of the plating and the maximum displacement.

Another problem associated with plating conventional spring structuresis the formation of “resist-edge” plating that is often undesirablydeposited around the springs close to the resist mask that defines therelease window. A resist-reflow step (e.g. resist annealing, acetonereflow) is often used to avoid the resist-edge plating, but the reflowstep does not always reliably prevent the formation of resist-edgeplating, and it is also difficult to implement in production.

Accordingly, what is needed is a cost effective method for fabricatingspring probes and other spring structures from self-bending springmaterials that are thick (stiff) enough to support, for example, largeprobing forces, but avoid the delamination associated with conventionalplated spring structures. What is also needed is a cost effective methodfor fabricating probes and other spring structures that reliablyprevents the formation of resist-edge plating.

SUMMARY OF THE INVENTION

The present invention is directed to plated spring structures that avoidthe problems associated with conventional spring structures byincluding, in addition to the base (anchor) section and curvedcantilever section typically associated with conventional springstructures, an elongated section that extends from the base sectionunder the cantilevered section. This elongated section increases theeffective area of the spring structure base and precludes the formationof wedge structures and spontaneous plating depositions that serve asundesirable fulcrums to delaminate conventional spring structures, andalso eliminates the need for resist-reflow operations used to preventresist-edge plating in conventional spring structures.

In accordance with an embodiment of the present invention, the elongatedsection is formed at least in part from plated material that isdeposited at the same time as cladding layers are plated onto a releasedspring finger. The spring finger is formed, for example, from a suitableself-bending spring metal film (e.g., stress-engineered metal, or abimorph/bimetallic material) that is “released” using known techniquessuch that a fixed end (the “base” or “anchor portion”) of the springfinger remains attached to the underlying substrate, and the curved freeend (the “cantilevered section”) bends relative to (e.g., away from) thesurface of the substrate. During subsequent plating of the springfinger, in addition to plating portions formed on the fixed and freeportions of the spring finger, plating material is intentionally formeddirectly under the released spring finger to form (or enhance) theelongated section. That is, unlike conventional spring structures inwhich the formation of plating material under the spring finger isavoided, a spring structure formed in accordance with the presentinvention includes a plating portion that is intentionally formed in theelongated section (i.e., under the raised cantilevered section). Thiselongated section increases the mechanical strength of the springstructure because it serves to “cement” (secure) the base (anchorportion) of the spring finger to the underlying substrate. Inparticular, the elongated “cementation” section in effect 1) makes astrong anchor and 2) prevents the thickness of the spring near the basefrom getting too thick. When there were wedge problems (no cementationused), the inventors would get the fulcrum effect as well as a thickerbase of the spring—thicker than intended—so it would have higherstresses when compressed and fracture more readily. Further, theelongated section provides enhanced resistance to delamination byprecluding the formation of undesirable wedge structures and/or thespontaneous formation of deposited metal (i.e., because the spaceotherwise utilized by such delaminating structures is purposefullyfilled with the plating materials associated with the elongatedsection). For similar reasons, the cementation section avoids theformation of resist-edge plating structures. The elongated section alsoserves to decrease the electrical resistance of the compressed spring byproviding a larger conducting volume.

In accordance with another embodiment of the present invention, thesubstrate surface under the cantilevered section is pre-treated and/or aseed (“cementation”) layer is provided to promote the formation of theplating material associated with the elongated section. In one specificembodiment, the seed layer is formed under the self-bending film used toform the spring finger, and is exposed when the spring finger isreleased. This seed layer is then utilized during an electroplatingprocess to form the elongated section. In the second specificembodiment, the substrate surface below the release spring finger istreated to activate the area below the cantilevered spring foreletroless plating of a metal layer, which may then be used duringfurther electroless plating or electroplating to produce the elongatedsection.

According to another aspect of the present invention, the elongatedsection is used to connect its corresponding spring structure directlyto associated trace metal areas formed on the substrate, or by way ofvia structures extending through insulating layers or the substrateitself, thereby reducing the effective width and increasing the packingdensity of the spring structures.

In accordance with another aspect of the present invention, the basesection of a spring structure is formed with a width that issubstantially wider than that of the cantilever section, and in someinstances wider than the width of the elongated section. An advantage tothe wide base section is that the spring structure may be fabricatedusing a highly efficient fabrication process that obviates the need formasking the anchor portion of the spring finger during release, andallows the use of lithography masks that are designed in such a way thatno extra mask is needed for spring cementation (i.e., post releaseplating).

In accordance with yet another embodiment, the elongated section is usedas a back-side exposure mask to pattern material formed on the springtip.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a perspective view of a spring structure according to anembodiment of the present invention;

FIG. 2 is a cut-away perspective view of a spring structure according toa specific embodiment of the present invention

FIGS. 3(A), 3(B) and 3(C) are cross-sectional side views showingsimplified fabrication steps associated with the production of thespring structure shown in FIG. 1 according to an embodiment of thepresent invention;

FIGS. 4(A), 4(B) and 4(C) are cross-sectional side views showingsimplified fabrication steps associated with the production of thespring structure shown in FIG. 1 according to another embodiment of thepresent invention;

FIGS. 5(A) and 5(B) are side views showing spring structures accordingto alternative embodiments of the invention;

FIGS. 6(A) and 6(B) are cross-sectional end views showing springstructures according to alternative embodiments of the invention;

FIGS. 7(A) and 7(B) are top plan view showing a conventional springstructure arrangement and a spring structure arrangement according toanother embodiment of the present invention, respectively;

FIGS. 8(A), 8(B) and 8(C) are cross-sectional side views showingalternative connection structures associated with the spring structureshown in FIG. 7(B);

FIGS. 9(A), 9(B), 9(C), 9(D), 9(E), 9(F), 9(G), 9(H) and 9(I) are topviews showing fabrication steps associated with the production of aspring structure according to another embodiment of the presentinvention;

FIG. 10 is an enlarged photograph showing an actual spring structureproduced in accordance with an embodiment of the present invention;

FIGS. 11(A), 11(B), 11(C), 11 (D) and 11 (E) are cross-sectional sideviews showing fabrication steps associated with the production of aspring structure according to another embodiment of the presentinvention;

FIG. 12 is a side view showing a compressed spring structure formed inaccordance with the present invention;

FIG. 13(A) is a simplified side view showing a conventional downwardbending spring structure; and

FIGS. 13(B), 13(C) and 13(D) are simplified side views showing downwardbending spring structures according to additional embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a spring structure 100 according toan embodiment of the present invention. Spring structure 100 is formedon an upper surface 55 of a host substrate (e.g., a glass, quartz,silicon, ceramic or flexible substrate) 51. The term “substrate” alsoincludes any flexible or rigid substrate upon which integrated circuitshave been fabricated (e.g., a silicon wafer from an integrated circuitfoundry would have many transistors fabricated on the substrate, on thissubstrate the spring structure could be fabricated). Spring structure100 generally includes a base section 101, an elongated section 102, anda curved cantilever section 105. Base section 101 is attached to surface55 (or to an optional intervening layer-not shown), and extends to ajunction point 106. Elongated section 102 has a first end attached tobase section 101 at junction point 106, and extends away from basesection 101 in a direction parallel to surface 55 to a free end 103.Curved cantilever section 105 has a fixed end attached to base structure101 at junction point 106, and gradually curves away from base structure101 to a tip (free end) 107 such that an angled air gap 109 is definedbetween an upper surface 104 of elongated section 102 and a lowersurface 108 of curved cantilever section 105. The phrase “angled airgap” in this context means that air gap 109 has relatively smallintermediate value G_(Int) adjacent to junction point 106 and basesection 101, and a relatively large value G_(Max) adjacent to free end107.

FIG. 2 is a cross-sectional side view showing spring structure 100 inadditional detail. Spring structure 100 includes an optional cementationlayer 110 formed on upper surface 55 (or on an intermediate layer, notshown, that is formed between upper surface 55 and cementation layer110), a spring finger 120 formed over cementation layer 110 (or anintervening layer formed between cementation layer 110 and spring finger120), and a cladding layer (plated metal structure) 130 formed oncementation layer 110 and spring finger 120.

Optional cementation layer 110 includes a first end portion 111 locatedin base section 101, and a second end portion 112 located in elongatedsection 102, and is entirely formed on or over surface 55. In oneembodiment, cementation layer 110 is a suitable plating seed layer(e.g., gold (Au)) that is formed on a region of substrate 55 forpurposes of promoting the formation of cladding layer 130 byelectoplating. Cementation layer 110 may also be selected from materialssuitable for promoting the formation of cladding layer 130 byelectroless plating, and may be omitted entirely in some embodiments.

Spring finger 120 includes an anchor portion 121 located in base section101, and a curved free portion 125 that extends from anchor portion 121and is located in curved cantilever section 105. According to an aspectof the present invention, spring finger 120 is fabricated using one ormore self-bending spring metals (e.g., stress-engineered metals orbimorph/bimetallic compositions) that facilitate selective andcontrollable bending of the spring structure. The phrase “self-bendingspring metal” is defined herein as a metal film having a non-zerointernal mechanical stress gradient when formed or subsequently annealedthat causes the metal film to bend (curl) away from the substrate afterrelease. The term “stress-engineered metal” or “stressy metal” isdefined herein as a sputtered or plated metal film either including anon-zero internal stress gradient, or an intermetallic metal film formedin accordance with co-owned and co-pending U.S. patent application Ser.No. 10/912,418, entitled “Intermetallic Spring Structure”, which isincorporated herein by reference. Spring metals may include non-metalcomponents.

Cladding layer 130 is a plated metal layer formed over spring finger 120and optional cementation layer 110, and includes a base (first) platingportion 131 formed over base section 101, an extended (second) platingportion 132 formed over elongated section 102, and a cantilevered(third) plating portion 135 formed over cantilever section 105. Notethat extended plating portion 132 is integrally joined to base platingportion 131 and cantilevered plating portion 135 at a junction region136, and extends from base plating portion 131 under cantileveredplating portion 135. Cladding layer 130 is formed using known platingtechniques (e.g., electroplating and/or electroless plating), and is atleast partially formed using one or more metals (e.g., one or more ofcopper (Cu), nickel (Ni), rhodium (Rh), palladium (Pd), cobalt (Co),chromium (Cr), silver (Ag), zinc (Zn), iron (Fe), cadmium (Cd) and gold(Au)).

Spring structure 100 is distinguished over conventional springstructures in the purposeful formation of elongated section 102 byplating material that is deposited during the formation of claddinglayer 130. That is, conventional spring structure fabrication processestypically involve plating the cantilevered and/or base section, but takeprecautions to avoid the formation of plated metal under thecantilevered section for reasons discussed above (i.e., the formation of“wedge” structures greatly increase the likelihood of delamination).According to an aspect of the present invention, the formation of platedmetal under the cantilevered section is not only tolerated, it is infact stimulated such that extended plating portion 132 is formed undercantilevered plating portion 135. The resulting plating structure formedby base plating portion 131 and extended plating portion 132 serves to‘cement’ anchor portion 121 of spring finger 120 to substrate 51, whichprovides spring structure 100 with a significantly greater adhesivestrength over conventional spring structures. In effect, elongatedsection 102 enlarges the base section 101 such that the point at whichcantilevered section 105 separates from the underlying structure (i.e.,junction point 106) is shifted to the right (with reference to FIG. 1).Further, elongated section 102 provides enhanced resistance todelamination by precluding the formation of undesirable wedge structuresand/or the spontaneous formation of deposited metal (i.e., because thespace otherwise utilized by such delaminating structures is purposefullyfilled with elongated plating portion 132). Thus, compared toconventional spring structures, spring structure 100 exhibits superiorresistance to delamination.

As indicated above, elongated section 102 is substantially formed byelongated plating portion 132, which is formed at least partially byplating material that is deposited simultaneously with base platingportion 131 and cantilevered plating portion 135. As set forth in thefollowing exemplary embodiments, the formation of elongated platingportion 132 is stimulated either by providing cementation layer 110prior to depositing the self-bending film used to form spring finger120, or by treating the portion of substrate surface 55 located belowcurved free portion 125 before the plating process.

FIGS. 3(A), 3(B), and 3(C) depict a fabrication process for producingspring structure 100 according to an embodiment of the present inventionin which cementation layer 110 is utilized.

As shown in FIG. 3(A), fabrication begins by sequentially forming and/orpatterning optional cementation layer 110 and a spring material island310 using known lithographic techniques. In one embodiment, cementationlayer 110 includes gold (Au) or another suitable seed material (e.g.,nickel (Ni) and/or copper (Cu)) deposited to a suitable thickness (e.g.,10-100 nm). Spring material island 310 is formed using a selectedself-bending spring metal and, although not shown, one or moreintermediate layers (e.g., a sacrificial “release” layer) may be formedbetween cementation layer 110 and spring material island 310.

In one embodiment, the self-bending spring metal used to form springmaterial island 310 is a stress-engineered film in which its lowermostportions (i.e., the deposited material adjacent to cementation layer110) has a lower internal tensile stress than its upper portions (i.e.,the horizontal layers located furthest from cementation layer 110),thereby causing the stress-engineered metal film to have internal stressvariations that cause a spring metal finger to bend upward away fromsubstrate 51 during the subsequent release process. Methods forgenerating such internal stress variations in stress-engineered metalfilms are taught, for example, in U.S. Pat. No. 3,842,189 (depositingtwo metals having different internal stresses) and U.S. Pat. No.5,613,861 (e.g., single metal sputtered while varying processparameters), both of which being incorporated herein by reference. Inone embodiment, which utilizes a 0.05-0.2 micron titanium (Ti) releasematerial layer, a stress-engineered metal film includes one or more ofmolybdenum (Mo), a “moly-chrome” alloy (MoCr), tungsten (W), atitanium-tungsten alloy (Ti:W), chromium (Cr), copper (Cu), nickel (Ni)and a nickel-zirconium alloy (NiZr) that are either sputter deposited orplated over the release material in the manner described above to athickness of 0.3-2.0 micron. An optional passivation metal layer (notshown; e.g., gold (Au), platinum (Pt), palladium (Pd), or rhodium (Rh))may be deposited on the upper surface of the stress-engineered metalfilm to act as a seed material for the subsequent plating process if thestress-engineered metal film does not serve as a good base metal. Thepassivation metal layer may also be provided to improve contactresistance in the completed spring structure. In an alternativeembodiment, a nickel (Ni), copper (Cu) or nickel-zirconium (NiZr) filmmay be formed that can be directly plated without a seed layer. Ifelectroless plating is used, the deposition of the electrode layer canbe skipped.

In an alternative embodiment, the self-bending spring material used toform spring island 310 may be one or more of a bimorph/bimetalliccompound (e.g., metal1/metal2, silicon/metal, silicon oxide/metal,silicon/silicon nitride) that are fabricated according to knowntechniques.

As indicated in FIG. 3(B), the fabrication process includes releasing(actuating) the self-bending spring metal of the spring metal island toform spring finger 120. When the self-bending spring metal used to formspring finger 120 is a stress-engineered metal film, the releasingprocess may involve, for example, masking anchor portion 121, and thenunder-etching the exposed free portion 125, thereby releasing(separating) free portion 125 from the underlying substrate 51. Uponrelease, free portion 125 bends into a curved shape in a manner thatrelieves its internal stress gradient (note that this stress gradient isretained in the anchor portion). Note that anchor portion 121 remainsfixed to substrate 51 by way of an intervening section of the releasematerial layer (when used, not shown) and/or section 111 of optionalcementation layer 110 (when used). Note also that the release process isperformed such that portion 112 of cementation layer 110 is exposedunder free portion 125 of spring finger 120 after the release process iscompleted. Alternatively, or in addition, the releasing process mayinvolve heating/annealing free portion 125 at a suitable temperatureuntil the desired curvature is achieved. For example, when theself-bending spring metal includes a bimorph/bimetallic compound, therelease process may be entirely performed by annealing, or may bereleased by selective delamination (the stress-gradient in the springovercomes the adhesion to the underlying substrate surface).Alternatively, when plated stress-engineered films are used, the releaseprocess may include both under-etching and annealing. Accordingly, thepresent invention is not limited to a particular process and/orself-bending material utilized to produce spring finger 120 unlessotherwise specified in the appended claims.

Finally, as depicted in FIG. 3(C), plating structure 130 iselectroplated or electroless plated over spring finger 120 and portion112 of cementation layer 110, thereby completing the production ofspring structure 100. Note that as the electroplating process proceeds,a junction plating portion 136 is formed, for example, in the V-shapedregion defining the point of separation of spring finger 120 (i.e.,under free portion 125 adjacent to anchor portion 121). As indicated inFIG. 3(C), plating portion 136 increases the size of the springstructure base by shifting the point of separation from an originallocation L1 (i.e., where spring finger 120 separates from cementationlayer 110) to location L2 (i.e., where cantilevered plating portion 135separates from elongated plating portion 131). Note also that theeffective point of separation (at location L2) is shifted upward fromthe plane separating anchor portion 121 and portion 111 of cementationlayer 110 by the thickness of elongated plating portion 131. In thismanner, anchor portion 121 of spring finger 120 is securely cemented(i.e., embedded in and/or surrounded by plating material) to underlyingsubstrate 51.

FIGS. 4(A), 4(B), and 4(C) depict a fabrication process for producingspring structure 100 according to another embodiment of the presentinvention in which a cementation layer is not utilized. In this case,the cementation area (i.e., the substrate surface on which the elongatedsection is formed) might also be non-conductive (e.g., benzo cyclobutene (BCB), polyimide, oxide, nitride). The present inventors observedin experiments that such a non-conductive surface can be metallizedusing electroless plating, and that this can be done at the same time asthe spring is overplated using the process shown in FIGS. 4(A) to 4(C).As depicted in FIG. 4(A), a spring metal island 410 is formed oversurface 55 of substrate using any of the above-mentioned self-bendingspring metal (e.g., stress-engineered metal, bimorph/bimetallic) films.As indicated, in FIG. 4(B), the spring island is then released using themethods described above to produce spring finger 120. Subsequent torelease, with the release mask still in place, a cementation area 55A ofsubstrate 51 (i.e., the area that is located under free portion 125 ofspring finger 120) is pre-treated using, for example, a solution ofstannous chloride and/or palladium chloride to activate thenon-conductive substrate material for electroless plating. As indicatedin FIG. 4(C), electroless plating of a selected plating material 420(e.g., NiP or NiB) is then used to deposit metal on both spring finger120 and cementation area 55A. The plating process can then either becontinued with electroless plating or electroplating to finish platingportions 131, 132 and 135 of plating structure 130. Note that thisspring cementation process is especially attractive because it can beperformed without requiring any additional lithography steps and masks.This means that spring cementation can be added to existing springtechnologies (e.g. stressed-metal, bimorph/bimetallic) simply by addingthe electroless plating step described above, and once a thin conductinglayer has been obtained, electroplating can be used to achieve thedesired plating thickness.

FIGS. 5(A), 5(B), 6(A) and 6(B) illustrate various optional featuresassociated with spring structures formed in accordance with the presentinvention.

FIGS. 5(A) and 5(B) indicate that the length of the elongated sectionrelative to the length of the cantilevered section may be varied. Thatis, in addition to being substantially equal in length to cantileveredsection 105 (e.g., as indicated in FIG. 1), elongated section 102 may beshorter or longer than cantilevered section 105. For example, asindicated in FIG. 5(A), spring structure 100-B1 includes a curvedcantilever section 105 having a tip 107 that is located a distance D1from base section 101 (e.g., from junction point 106), and an elongatedsection 102-B1 having an end 103-B1 that is located a distance D-B1 frombase section 101, where distance D-B1 is less than distance D1.Advantages of providing the relatively short elongated section 102-B1are discussed below. Conversely, as indicated in FIG. 5(B), springstructure 100-B2 includes curved cantilever section 105, the length D1,and an elongated section 102-B2 having an end 103-B2 that is located adistance D-B2 from base section 101, where distance D-B2 is greater thandistance D1. The length of elongated section 102-B2 may be adjusted inthis manner, for example, to provide connection to other structuresformed on the substrate.

FIGS. 6(A) and 6(B) indicate that the width of the elongated sectionrelative to the width of the cantilevered section may also be varied,and may cover two or more spring structures. As indicated in FIG. 6(A),both elongated section 102-C1 and cantilevered section 105-C1 of springstructure 100-C1 have substantially (i.e., within 10%) the same widthW-C1. In contrast, spring structure 100-C2 includes a cantileveredsection 105-C2 having width W-C1 and an elongated section 102-C2 havinga width W-C2 that is smaller than width W-C1, and spring structure100-C3 includes a cantilevered section 105-C3 having width W-C1 and anelongated section 102-C3 having a width W-C3 that is substantiallygreater than width W-C1. Finally, as indicated in FIG. 6(B), severalspring structures 100-D1 to 100-D4 may include correspondingcantilevered sections 105-D1 to 105-D4, each having a width W-D1, and asingle elongated section 102-D having a width W-D2 that spans all fourspring structures.

Another advantage associated with the present invention is that theelongated section may be used to connect the corresponding springstructure to associated trace metal areas formed on the substrate,thereby reducing the effective width and increasing the packing densityof the spring structures. As indicated in FIG. 7(A), in order to connectconventional springs 70-1 to 70-3, which are aligned in parallel in anX-direction, to corresponding trace areas 80-1A to 80-3A, which are alsoaligned in the X-direction, short trace segments 81 extending in theY-direction (i.e., away from the associated spring structure) must beused, thereby resulting in a relatively wide spring structure pitch P1.In contrast, as indicated in FIG. 7(B), by utilizing elongated sections102 (which are located under corresponding cantilevered sections 105),and in particular relatively long elongated sections such as those shownin FIG. 5(B), to connect spring structures 100-E1 to 100-E4 to co-lineartrace areas 80-1B to 80-4B, a relatively narrow spring structure pitchP2 is enabled that increases the spring structure packaging density.

FIGS. 8(A) through 8(C) show various trace metal connection arrangementsthat are facilitated by spring structures formed according to thepresent invention. FIG. 8(A) shows a simple arrangement in which a tracestructure 80-F1 is formed on or over surface 55-F1 of a substrate 51-F1,and contacts elongated section 102 of spring structure 100-F1, which isformed as described above. FIG. 8(B) shows a second arrangement in whichone or more trace structures 80-F2 are embedded in an insulating layer90, which is formed on a surface 55-F2 of a substrate 51-F2 according toknown techniques, where the uppermost trace structure 80-F2 is connectedto elongated section 102 of spring structure 100-F2 by a via structure82-F2 that extends through insulating layer 90. FIG. 8(C) shows a thirdpossible arrangement in which a trace structure 80-F3 is formed on alower surface 57-F3 of a substrate 51-F3, and is connected to elongatedsection 102 of spring structure 100-F3 by a via structure 82-F3 thatextends through substrate 51-F3. The arrangements shown in FIGS. 8(B) to8(C) provide structures in which no extra space needed for trace metal,and hence a very high spring density is possible. The embodiments shownin FIGS. 8(A) to 8(C) are intended to be exemplary, and are not intendedto limit the appended claims unless otherwise specified.

According to another aspect of the present invention, the base area ofthe spring structures are formed with widths that are substantiallygreater than the widths of the cantilever sections, and in someinstances may be greater than the width of the elongated platingsection. An advantage to wide base sections is that spring structuresmay be fabricated using a highly efficient fabrication process thatobviates the need for masking the base section during release (that is,because the base is substantially wider than the cantilevered section,the base remains securely attached to the substrate during the releaseprocess). This also allows the use of lithography masks that aredesigned in such a way that no extra mask is needed for springcementation (i.e., post release plating). One such efficient processflow is described below with reference to FIGS. 9(A) to 9(I).

Referring to FIG. 9(A), a first mask is used to form a resist island1115 on a layer 1110 of cementation material. The exposed cementationmaterial is then removed (e.g., etched) to expose upper surface 55 ofsubstrate 51, and then the resist island is removed to exposecementation layer 110 (FIG. 9(B)). FIG. 9(C) depicts the formation of aresist layer 1120 around cementation layer 110, and this mask is used topattern the self-bending spring metal 910 over cementation layer 110(FIG. 9(D)). As shown in FIG. 9(E), resist layer 1120 and anyself-bending material formed thereon is then removed using knownlift-off techniques, and then another mask 1130 is formed around thelayered stack formed by cementation layer 110 and self-bending springmaterial island 910 (FIG. 9(F)). The self-bending spring material islandis then released to form spring finger 120 (FIG. 9(G)), and then platingstructure 130 is formed over spring finger 120 and the exposed portionof cementation layer 110 (FIG. 9(H)). Finally, the release/plating maskis removed (FIG. 9(I)) to complete the fabrication of spring structure100-G1. Note that by forming base section 101-G1 with a width W-G1 thatis substantially greater than the width W-G2 of cantilever section120-G1, base section 101-G1 remains secured to substrate 51 during therelease process.

FIG. 10 is an enlarged photograph showing a spring structure 100-G2formed in accordance with the method described above with reference toFIGS. 9(A) to 9(I). Note that base section 101-G2 is substantially widerthan both elongated section 102-G2 and cantilevered section 105-G2.

Several additional alternative embodiments and applications of thepresent invention are described below.

According to an alternative embodiment, the cementation (plating)process is used in combination with self-releasing springs and springencapsulation. In case of self-releasing springs that utilize selectivedelamination (mentioned above), the self-release area can also serve asspring cementation area. For encapsulated springs, which utilize anencapsulation layer to isolate the spring structure during release etch,spring cementation can be implemented as for common springs.

FIGS. 11(A) to 11(E) depict another alternative embodiment that uses theelongated section and back-side exposure to pattern both sides of thespring tip. Patterning the spring tip on both sides is commonlydifficult to do but it is very interesting for certain applications(e.g. solder stop for interconnect, selective tip coating forbio-applications). Referring to FIG. 11(A), the present embodimentbegins using spring structure 100-B1, which is described above withreference to FIG. 5(A), where cantilevered section 105-B1 extendsfurther from base 101-121 than elongated section 102-B1. A resistcoating 1110 is deposited over base section 101-121, elongated section102-B1 and cantilevered section 105-B1 according to known techniques(FIG. 11(B)). Next, a top side shadow mask 1120 defining a window 1115is used to expose the upper side of tip 107-B1 of cantilevered section105-B1 (FIG. 11(C)), and then base section 101-B1 and elongated section102-B1 are utilized as a backside mask (i.e., to block beams passedthrough lower surface 57 of substrate 51) to expose the lower surface oftip 107-B1 (FIG. 11(D)). In this way, tip 107-B1 can be patterned onboth sides, and resist material 1110 can be selectively removed justfrom tip 107-B1 by etching (as indicated in FIG. 11(E)) and/or materialcan be deposited onto tip 107-B1 using known techniques (not shown).

As set forth by the specific embodiments described above, the presentinvention introduces an elongated “cementation” section into springdevices for increasing the strength of the spring base (anchor), and forovercoming the problem of resist-edge plating and spontaneous metaldeposition under the spring. In addition, as indicated in FIG. 12,elongated section 102 improves conductivity of the compressed springstructure 100 by increasing the total metal volume, and/or by decreasingthe length of the electrical path between tip 107 and base 101 due tothe contact between the flattened portion of cantilevered section 105and elongated section 102. That is, when an object 1200 (e.g., anintegrated circuit device-under-test) presses downward on tip 107,cantilever section 105 is flattened (bent toward substrate 51), whichcauses the effective junction point to move from initial point 106A to asecond point 1061, thereby reducing the distance signals are required topass along cantilever section 105. Note that this feature is especiallyattractive for relatively thin springs, and further facilitates the useof under-spring trace patterns, via structures and through-substrateinterconnects positioned directly under the spring, as described above.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention. For example, although the presentinvention is specifically directed to plating formed on spring metalstructures, plating may also be formed on non-metal spring structures(e.g., using a bilayer of oxide and silicon or Ga—As covered by a metalseed layer) using, for example, electroless plating techniques.Moreover, although the present invention describes spring structuresthat bend away from an underlying substrate, the present invention mayalso be utilized in spring structures in which the tensile/compressivestress gradient is reversed, causing the released spring structure tobend toward the substrate (e.g., into a trench formed in the substrate).FIG. 13(A) shows a spring structure 100-13A including a spring finger120-13A having a negative gradient (for example low tensile stress ontop, high tensile stress on bottom), such that free portion 125-13Acurves down when released. In this case, a portion of substrate 51 isremoved so that the spring is allowed to curl down. FIG. 13(B) shows astructure 100-13B according to an embodiment of the invention whichincludes an essentially identical spring finger 120-13B as that shown inFIG. 13(A), but this case shows the substrate 51 not removed from underthe released spring so a middle section of free portion 125-13B ofspring finger 120-13B pops up, but tip 128-13B stays down. The mainadvantage of spring structure 100-13B over spring structure 100-13A isthat spring structure 100-13B can be contacted from the side, not justthe top. A force from a sliding metal pad from the right (tip side) willcompress the spring and make a good electrical contact. This isespecially important for connector applications. FIG. 13(C) shows aspring structure 100-13C according to another embodiment of the presentinvention that includes a base section 101-13C and a cantilever section105-13C similar to the structure shown in FIG. 13(B), but also includesan elongated section 102-13C extending under cantilever section 105-13Cin such a way that it strengthens junction point 106-13C of cantileversection 105-13C. Junction point 106-13C needs to be strong and ofcontrolled thicknesses so that spring structure 100-13C can operate asdesigned for reasons similar to those described above. As indicated bythe dashed line, when cantilevered section 105-13C is compressed asshown by a force F, tip 107-13C slides along the substrate surface tothe right. With a well-designed anchor this springs would have a largecompliance range. FIG. 13(D) shows a spring structure 100-13D that issimilar to structure 100-13C, but plated elongated section 102-13Dlocated under cantilever section 105-13D extends beyond tip 107-13D.Elongated section 102-13D plates to tip 107-13D and secures it so tip107-13D doesn't slide when spring structure 100-13D is compressed (asindicated by dashed line). Spring structure 100-13D would sustain veryhigh forces F because it is doubly clamped, but would have a smallercompliance range than spring structure 100-13C.

Further, although the present invention is described with reference tospring structures having in-plane tips, the present invention may alsobe utilized in spring structures having out-of-plane tip structures.

1. A spring structure formed on a substrate having a first surface, thespring structure comprising: a base section attached to the firstsurface; and a curved cantilever section having fixed end attached tothe base section, a middle section disposed a first distance away fromthe first surface of the substrate, and a tip end disposed in slidablecontact with the first surface of the substrate, wherein both the basesection and the curved cantilever section comprise one of astress-engineered spring material structure, and a bimorph/bimetallicstructure.
 2. A spring structure formed on a substrate having a firstsurface, the spring structure comprising: a base section attached to thefirst surface; a curved cantilever section having fixed end attached tothe base section, a middle section disposed a first distance away fromthe first surface of the substrate, and a tip end disposed in slidablecontact with the first surface of the substrate; and a plated materialdisposed on the base section and the curved cantilever section.
 3. Thespring structure according to claim 2, further comprising an elongatedsection attached to the first surface and extending from the fixed endunder the middle section of the curved cantilever section such that anair gap is defined between the middle section and the elongated section.4. The spring structure according to claim 3, wherein the tip end of thecurved cantilever section abuts the elongated section.
 5. A springstructure formed on a substrate having a first surface, the springstructure comprising: a base section attached to the first surface; anelongated section having a first end attached to the base section, theelongated section extending away from the base section in a directionparallel to the first surface; and a curved cantilever section having afixed end attached to the base section, a curved middle sectionextending away from the substrate and then back down toward thesubstrate, and a second end disposed at an end of the middle section,wherein the elongated section is located between the middle section ofthe curved cantilever section and the first surface of the substrate,and between the fixed and second ends of the curved cantilever section,wherein an air gap is defined between the elongated section and thecurved middle section of the curved cantilever section, and wherein thespring structure comprises one of a stress-engineered spring materialstructure, and a bimorph/bimetallic structure.
 6. The spring structureaccording to claim 5, further comprising a plated metal materialdisposed on the base section and the curved cantilever section.
 7. Thespring structure according to claim 5, wherein the elongated sectionextends from the fixed end to the second end of the curved cantileversection, and the second end of the curved cantilever section is attachedto the elongated section.
 8. A spring structure formed on a substratehaving a planar surface, the spring structure comprising: a springfinger having an anchor portion secured to the planar surface, and acurved free portion having a first section extending from the anchorportion away from the planar surface, and a second section extendingfrom the first section toward the substrate and having a tip; and aplated metal structure including a base plating portion formed on theanchor portion of the spring finger, and an elongated plating portionextending from the base plating portion in a direction substantiallyparallel to the first surface and positioned such that the elongatedplating portion is located between the curved free portion of the springfinger and the planar surface of the substrate and the tip of the springfinger, wherein an air gap is defined between the elongated section andthe curved free portion of the spring finger, and wherein the springfinger comprises one of a stress-engineered spring material structure,and a bimorph/bimetallic structure.
 9. The spring structure according toclaim 8, wherein the tip of the spring finger abuts at least one of theelongated plating portion and the planar surface of the substrate. 10.The spring structure according to claim 8, wherein the tip of the springfinger is connected to the elongated plating portion.
 11. The springstructure according to claim 8, wherein the elongated section extendsfrom the fixed end to the second end of the curved cantilever section,and the second end of the cured cantilever section is attached to theelongated section.
 12. The spring structure according to claim 11,wherein the spring structure further comprises a cementation layerhaving a first end portion located in the base section, and a second endportion located in the elongated section.
 13. The spring structureaccording to claim 12, wherein the cementation layer comprises at leastone of gold (Au), nickel (Ni), and copper (Cu), and the plated metalstructure comprises at least one of copper (Cu), nickel (Ni), rhodium(Rh), palladium (Pd), cobalt (Co), chromium (Cr), silver (Ag), zinc(Zn), iron (Fe), cadmium (Cd), gold (Au), nickel-phosphor (NiP), andnickel-boron (NiB).
 14. A spring structure formed on a substrate havinga planar surface, the spring structure comprising: a spring fingerhaving a first anchor portion secured to the planar surface, and acurved free portion having a first section extending from the anchorportion away from the planar surface, and a second section extendingfrom the first section to the substrate to form a second anchor portionthat is secured to the planar surface of the substrate; and a platedmetal structure including a base plating portion formed on the anchorportion of the spring finger, and an elongated plating portion extendingfrom the base plating portion in a direction substantially parallel tothe first surface and positioned such that the elongated plating portionis located between the curved free portion of the spring finger and theplanar surface of the substrate, wherein the elongated plating portionis connected to the second anchor portion of the spring finger, whereinthe spring finger comprises one of a stress-engineered spring materialstructure, and a bimorph/bimetallic structure.